A wide variety of semiconductor devices, and methods of making semiconductor devices, are known. Some of these devices are designed to emit light, such as visible or near-visible (e.g. ultraviolet or near infrared) light. Examples include light emitting diodes (LEDs) and laser diodes. Other devices are designed to detect light. Examples include semiconductor photodiodes and charge coupled devices (CCDs). Still other devices are designed to neither detect nor emit light, but to provide electronic signal functionality. Examples include semiconductor diodes, transistors, and integrated circuit devices.
A re-emitting semiconductor construction (RSC) is an example of a semiconductor device designed to emit light. Unlike an LED, the RSC does not require an electrical drive current from an external electronic circuit in order to emit light. Instead, the RSC generates electron-hole pairs by absorption of light at a first wavelength λ1 in an active region of the RSC. These electrons and holes then recombine in potential wells in the active region to emit light at a second wavelength λ2 different from the first wavelength λ1, and optionally at still other wavelengths λ3, λ4, and so forth depending on the number of potential wells and their design features. The initiating radiation or “pump light” at the first wavelength λ1 is typically provided by a blue, violet, or ultraviolet emitting LED coupled to the RSC. Exemplary RSC devices, methods of their construction, and related devices and methods can be found in, e.g., U.S. Pat. No. 7,402,831 (Miller et al.), U.S. Patent Application Publications US 2007/0284565 (Leatherdale et al.) and US 2007/0290190 (Haase et al.), PCT Publication WO 2009/048704 (Kelley et al.), and pending U.S. Application Ser. No. 61/075,918, “Semiconductor Light Converting Construction”, filed Jun. 26, 2008, all of which are incorporated herein by reference.
When reference is made herein to a light at a particular wavelength, the reader will understand that reference is being made to light having a spectrum whose peak wavelength is at the particular wavelength.
The RSC is typically fabricated by growing a series of semiconductor layers atop a semiconductor substrate using epitaxial growth techniques. The substrate material is selected such that it can provide a surface that has at least an approximate lattice match to the crystal lattice of the device layers to be grown. This selection criterion may require a semiconductor substrate whose crystal matrix incorporates indium. One known substrate used in the fabrication of RSCs is indium phosphide (InP). In some cases, the substrate includes a thin buffer layer grown atop a thick base layer, the buffer layer having a different chemical structure than the base layer and providing a pristine surface from which to begin the growth of the device layers. An exemplary buffer layer used in the fabrication of RSCs is gallium indium arsenide (GaInAs) grown atop a base layer of InP.
A vertical cavity surface emitting laser (VCSEL), which can be considered to be a type of RSC, is another example of a semiconductor device designed to emit light. The VCSEL converts at least a portion of a first wavelength light emitted by a pump light source, such as a III-V based pump light source that includes nitrogen, to at least a partially coherent light at a second wavelength. The VCSEL includes first and second mirrors that form an optical cavity for light at the second wavelength. The first mirror is substantially reflective at the second wavelength and includes a first multilayer stack. The second mirror is substantially transmissive at the first wavelength and partially reflective and partially transmissive at the second wavelength. The second mirror includes a second multilayer stack. The VCSEL further includes a semiconductor multilayer stack that is disposed between the first and second mirrors and converts at least a portion of the first wavelength light to the second wavelength light. The semiconductor multilayer stack includes a quantum well that includes a Cd(Mg)ZnSe alloy. Reference is made to pending U.S. Patent Application Ser. No. 61/094,270, “Diode-Pumped Light Source”, filed Sep. 4, 2008, incorporated herein by reference.